1. Field of the Invention
The present invention relates to process control systems, and more particularly, to pressure transmitters.
2. Description of the Related Art
There is a strong desire to detect plugged impulse lines in petrochemical plants and refineries using only the information available at a standard differential pressure cell (DP-cell). Many approaches to this problem use various statistical analysis techniques, such as wavelet processing or fast Fourier transforms to examine the higher frequency signals that are normally present on the differential pressure signal and determine if one or both of the inlet ports to a transmitter is plugged. The desired signal processing capability requires computational power and sampling rates that are well beyond what the standard differential pressure transmitter is able to achieve. This is because the standard differential pressure transmitter is extremely limited in its use of electrical power. The need for high sample rates to properly resolve the high frequency information increases the power required in the measuring instrument. There is a need for an improvement to a control system and method for detecting plugging in differential pressure cells that reduces the required sampling rate.
The time scale of plugging is generally much larger than the time scale of process condition variations. In applications where flow is measured, the process condition varies rapidly and the information relevant to determine an imminent plugged condition is at frequencies greater than 100 Hz. To gather this information in a manner consistent with the low power constraints imposed on these devices, the present inventors have discovered that a substantial improvement can result by decreasing the sampling rate significantly by taking samples at random time intervals. Turning to the present invention, samples are taken at a lower rate, taken at random times, and reconstructed into time scales suitable for processing. The present invention greatly reduces the sampling rate and also improves the information quality. First, samples taken over a longer period of time reduce the data-corrupting effects of small perturbations in samples taken over short periods. Second, taking samples at a lower rate and then reconstructing the original signal from fewer data points also reduces the required sampling rate. Third, random sample intervals are less correlated to periodic fluctuations in a process system. The present inventors have discovered that this is important because the control system and method for detecting plugging in differential pressure cells depends on the process noise and the flow noise being non-correlated. Changing from fixed sampling intervals to random sampling intervals improves data quality by reducing the possibility of having periodic disturbances in the process conditions alias the apparent process information.
One embodiment of the present invention is a method for detecting plugging in differential pressure cells. Output of a differential pressure transmitter is sampled at random intervals that are then reconstructed into a time varying output that represents the process signal to produce samples and plugging is detected based on the samples. In another embodiment, the z-statistic is tested for a zero velocity condition. A variance of said samples is calculated. A baseline value is updated based on the variance. A variance factor is developed by comparing the samples with the updated baseline value. It is determined whether the variance factor exceeds a threshold. A frozen sensor alert is provided when the zero velocity condition is met or the variance factor exceeds the threshold. In still another embodiment, a window is constructed from the samples and a variance of the samples in the window is calculated from the mean and the previous mean. In yet another embodiment, a mean of the samples is calculated. A z-statistic is generated from the mean and a previous mean.
These and other features, aspects, and advantages of the present invention will become better understood with reference to the drawings, description, and claims.